The screws are secured with a thread-locker compound. It sometimes takes considerable torque to loosen them, and if the screwdriver you use isn't good quality or the proper size, you can strip the head.
First try carefully heating the screw by touching a hot soldering iron or other hot object to the screwdriver shaft while pressed against the screw. Let it warm for about five seconds and then try turning - be sure to apply as much downward pressure on the screwdriver as you can.
If the head gets stripped, the usual fix is to carefully grind a slot on the head with a Dremel tool and use a flat-blade screwdriver to remove.